1. Field of the Invention
The present invention relates in general to flexible plastic tubing and, more particularly, to apparatus and methods for making double-walled flexible plastic tubing having a helical support bead and integrally incorporating at least one electrical resistance heating conductor. Still more particularly, the present invention relates to a flexible double-walled and electrically-heated plastic tubing having a greatly reduced rate of heat loss compared to conventional single-walled tubing of this type. The present tubing further is configured to maintain a substantially smooth bore even when bent.
2. Description of Related Art
Throughout this disclosure, it will be recalled that heat transfer in a body, or between the interior of a body and ambient (i.e., heat loss from within a body to ambient) may be considered either from the standpoint of heat transfer conductivity (C) (a measure of heat transfer quantity per unit of area per degree of temperature difference), or in terms of heat transfer resistivity (i.e., the familiar "R" rating frequently used on building insulation products, for example). The "R" value is simply the reciprocal of heat transfer conductivity (i.e., R=1/C). Thus, these concepts and values may be used interchangeably, with allowance for the reciprocal relationship of the values.
Conventional flexible, thin-walled, helically-corrugated tubing having a relatively thin single wall and an integral helical supporting bead is known in the art. Such support-bead tubing construction provides substantial crush resistance while leaving the tube wall flexible enough to permit short-radius bends without collapsing or kinking the tube. The versatility of this kind of tubing is evidenced by its wide applicability in construction, ventilation, manufacturing processes, auto washes, hospitals and other fields.
The wall of a support-bead tubing can be quite thin to minimize overall weight. The thin wall also improves flexibility of the tubing. This light weight and flexibility for the tubing is an important feature, for example, in an inhalator tube to provide a patient with more comfort during oxygen delivery. Two other features of known thin single-wall support-bead or bead-reinforced tubing are transparency and smoothness of bore. Transparent plastic material permits inspection of the fluid coursing through the tube, to detect, for example, the presence of moisture in an anesthetic or patient oxygen delivery application. A smooth inner surface of such a tube is desirable to keep the tube free from deposits of contaminants and to discourage non-laminar flow. That is, the smooth bore of the tube promotes a low rate of fluid pressure drop along a length of the tubing, for example, between a breathing assistance machine and patient using such a machine.
U.S. Pat. No. 3,910,808 to Carlson, assigned to the same assignee as this application, discloses apparatus for forming such thin-walled, flexible, crush resistant support-bead tubing having a single wall. Carlson discloses means for extruding a plastic strip having a longitudinal rib, and winding means for helically winding the strip about an axis to produce a corrugated flexible tubing having a smooth bore.
U.S. Pat. No. 5,454,061 to Carlson, also assigned to the same assignee as this application, discloses apparatus for forming single-walled electrically-heated, flexible, crush resistant support-bead tubing. This Carlson patent discloses means for extruding a plastic ribbon, and winding means for helically winding the strip about an axis to produce a thin-walled tube. Upon this thin-walled tube is helically applied at least one electrical resistance heating conductor, and a helical support bead. This tubing substantially satisfies the requirements of many applications which require or are enhanced by the presence of controlled heating of such tubing.
For example, neonatal patients, as well as patients in shock, or patients who are sustained on breathing equipment, are among those who benefit from gas flowing through heat-conditioned tubing. Such is the case because the breathing apparatus usually provided conditioned (i.e., humidified and warmed and possibly medicated and/or oxygenated) air for inhalation by the patient at a temperature close to body temperature. Conventional unheated tubing allows significant heat loss to ambient and cooling of the warmed inhalation air, to the detriment of critical or tenuous patients. cooling of the conditioned air may cause considerable condensation of added humidity within the conventional tubing. It is conventional to use a moisture trap in such apparatus for the purpose of insuring that the patient does not inhale a slug of condensed water from the tubing.
The tubing according to the Carlson '061 patent allows electrical resistance heating to be applied to the tubing to essentially offset this heat loss to ambient from the warmed tidal air flow within the tubing. When properly used, the warmed tidal air flow in the tubing will have an equilibrium condition between its heat loss through the single wall of the tube and heat addition from the heating conductor(s) of the tubing. Thus, the patient will receive warmed conditioned air at substantially the same temperature provided by the inhalation device.
Prior attempts to achieve heating of a tube include providing: a resistance element extending linearly along the tube's axis (U.K. Patent No. 1,448,473 to Grant); fabric tape secured to a wire and applied to the tube (U.K. Patent No. 897,292 to Reik); or resistive wire in a tubing with no crush-resistant or thin-walled features (U.S. Pat. No. 4,038,519 to Foucras; U.S. Pat. No. 4,553,023 to Jameson et al.; and U.S. Pat. No. RE 29,332 to Bilbro et al.). These tubing products and those to be described in more detail in this section have one or more characteristics inconsistent with use in a medical environment. For example, tubing which relies on an adhesive binding for the support bead may deteriorate as a result of repeated sterilization. Materials may be biomedically incompatible, and exteriors are invariably characterized by crevices adjacent to the support bead which can harbor particulate matter and microbes. In addition, the more separate and distinct steps that are required in producing such tubing, the greater the cost, complexity and potential for failure of the product in use.
Another heatable tubing is described by DE 42 44 493A1 to Eilentropp. The '493A1 patent is believed to describe a respiratory tube with a spirally ribbed outer surface upon which electric heating conductors are placed adjacent to the spiral ribs. The heating conductors may be glued on to the outer surface of the tube. The ribs may be formed as a separate smaller tube profile which is then glued to the outer surface of the respiratory tube in a spiral arrangement. With a respiratory tube according to the '493A1 publication, the resistance wires must be separately secured to the outer surface of the tube, requiring a separate manufacturing step. Also, the separate glue may not provide as secure an attachment of the heating conductors to the respiratory tube as would be desired. There is a distinct possibility of imperfect match between the tube and the glue, and the glue also presents a possibility of solvents being released in the medical environment. As previously mentioned, the glue may not endure sterilization as well as the tube itself.
Finally, U.S. Pat. No. 3,686,354, issued to Makin, is believed to provide a thin-walled, flexible, but helically-ribbed collapse-resistant hose for inhalation apparatus. An inner thin-walled flexible tubular member defines a helical groove to which is helically secured an outer heater cable. The electrical heating cable is round in cross-section and is bonded to the outer surface of the tubular member by adhesive or vulcanization. With the inhalation hose according to the '354 patent, the helical heater cable does not become an integral part of the inner tubular member, but instead lays in a helical groove of the inner tubular member, defining a helical crevice on each side of the heater cable. This crevice or pair of crevices may provide an area in which soil and bacteria can escape cleaning and sterilizing efforts. Also, the heat originating at the conductors of the heater cable must be conducted through not only the insulation on this cable but also through to the wall of the inner tube. In fact, these heating conductors would appear to be more directly coupled to the ambient air than to tidal air in the tube.
Another conventional expedient sometimes employed is to provide a heating cable in the configuration of a loop having its two ends terminated adjacent to the inhalation machine. This heating cable is pulled into the flexible tubing leading from the machine to a patient so that it is exposed directly to the tidal air flow in this tubing. Installation of this heating cable into an elongate length of the inhalation tubing requires use of a hooked flexible pulling tool, somewhat like an electrician's or plumber's snake. The electrical connections to this heating cable are terminated at an elbow fitting adjacent to the inhalation therapy machine. Cleaning of the inhalation tubing requires removal and separate cleaning of the heating cable.
An alternative conventional support-bead tubing product is known which has double thin and flexible walls, with one wall inward of a helical support bead, and the other wall outward of the bead. The two flexible walls of this conventional tubing are spaced apart by the support bead and cooperatively define a trapped air space within which the trapped air is substantially stagnant, and provides a desirably high insulation value. Even though this conventional double-walled tubing is an improvement in some uses over the conventional un-heated single-walled support-bead tubing, it does not entirely eliminate heat loss to ambient from warmed tidal air flow in the tubing. In this respect, the conventional double-walled support-bead tubing cannot provide essentially a zero heat loss from tidal air flow along a length of the tubing, as can the electrically heated tubing of Carlson, disclosed in his '061 patent.
No prior product, method of manufacture, or apparatus is known which provides a flexible, support-bead, double-walled, smooth-bore tube having a high insulation value between warmed tidal air and ambient, as well as an electrical resistance heating conductor extending helically along the length of the tube for offsetting heat loss to ambient from warmed tidal air flow in the tube.